This invention relates generally to digital data storage channels and devices. More particularly, the present invention relates to a method for reducing sequence detector pad length within a partial response maximum likelihood data channel.
In order to achieve higher recording densities, designers of magnetic recording channels have switched from analog peak detection techniques to sampled data detection techniques. In sampled data detection systems, the readback signal is filtered and sampled at a channel rate of 1/T, where T is the duration of a channel symbol. One such technique employs what is known as a partial response maximum likelihood (PRML) system. When PRML is employed, magnetic transition densities on the recording medium may be increased by as much as 20% to 30% over peak detection recording and playback methods, since PRML more robustly tolerates some transition pulse overlap (intersymbol interference) than can resolved with peak detection techniques. Also, in the process of peak detection, the readback signal is differentiated in order to locate signal zero crossover locations. Differentiation amplifies higher frequencies which contributes additional noise to, and increased errors in, the readback signal. The synchronous sampling process employed in PRML quantizes signal amplitudes at specific intervals throughout each readback transition interval T without requiring determination of zero crossings, thereby eliminating the differentiation step and resultant noise enhancement.
One widespread PRML system uses filters to equalize the readback signal to a partial response class 4 (PR4) signal. The discrete-time transfer function of a PR4 channel is (1xe2x88x92D)2, where D represents a unit-time delay operator with unit-time T. In an idealized PR4 channel, a noiseless output is equal to the input signal minus a version of the input signal delayed in time by 2T. In a practical PR4 channel, the output of the noisy partial response channel is sampled at the channel rate and detected using a sequence detector, such as a Viterbi detector. Typically, the Viterbi detector is designed for maximum-likelihood detection of the sampled partial response channel in additive, independent, and identically distributed Gaussian noise with zero mean.
While PR4ML channels have been widely used in magnetic recording and playback systems for data densities at or below two channel symbols per pulse width at half maximum amplitude (PW50/Txe2x89xa62.0) the PR4 spectnim has satisfactorily matched the magnetic recording channel. However, at normalized data densities above PW50/T=2.0, other partial response models have been discovered to provide a better match to the magnetic recording channel characteristics. These partial response models include EPR4 with a discrete-time transfer function of (1xe2x88x92D)(1+D)2 or (1+Dxe2x88x92D2xe2x88x92D3) and EEPR4 with a discrete-time transfer function of (1xe2x88x92D)(1+D)3 or (1+2Dxe2x88x922D3xe2x88x92D4). Other partial response models are also known, such as NPR having a unit pulse response of e.g. 7+4Dxe2x88x924D2xe2x88x925D3xe2x88x922D4.
Once a channel model is selected, a sequence detector may be fashioned. Sequence detectors frequently implement a version of the Viterbi algorithm. Typically the Viterbi detector is designed for maximum likelihood detection of the sampled partial response channel in additive, independent, and identically distributed Gaussian noise with zero mean. The Viterbi algorithm minimizes squared Euclidean distance between the sequence of noisy samples and all possible sequences of idealized noiseless samples in accordance with the particular channel model. The Viterbi algorithm is an iterative process of keeping track of the path with the smallest accumulated metric leading to each state. The metrics of all of the paths leading into a particular state are calculated and compared. Then, the path with the smallest metric is selected as a survivor path and the other pathsare discarded. In this manner all paths which are not part of the minimum metric path are systematically eliminated. The survivor path to each state is stored in a path memory. Given that the path memory is made sufficiently long, all of the selected survivor paths will diverge from a single path within the span of the path memory. The single path from which all the current survivor paths diverge is the minimum metric path. The Viterbi detector then traces back along the path memory to find the convergence state. The input sequence associated with the single minimum metric path then becomes the most-likely symbol output of the Viterbi detector.
A Viterbi detector does not attempt to decide whether a transition has occurred upon receipt of a readback sample or samples taken from a particular transition. Rather, samples are taken from the readback signal and equalized to the target channel model. The Viterbi detector then keeps a running tally of the error between the actual sample sequence and a correct sample sequence, i.e. a sequence that would be expected if the recording medium had been written with a particular sequence of transitions. One way of visualizing the Viterbi detector path memory is by way of a trellis diagram having plural states and plural paths leading from each state to other states. As analog-to-digital samples (yk) are fed into one end of the trellis, estimates of previous bits are put out at an opposite end of the trellis. An error metric is determined for each one of plural possible state transition sequences. As more samples come into the Viterbi detector, less probable transition sequences (paths) are eliminated, and by tracing back along the trellis a most likely path emerges as a convergent set of paths and enables a most-likely data decision to be made by the Viterbi detector.
The magnetic recording channel is not an ideal channel. Rather, noise, media defects, non-linear response of the playback element and other distracting influences may result in distortion of or error in the readback less than signal. Therefore, error events can, and do, occur. When sequence detection is employed, error events may result in a most likely path being selected by the Viterbi detector which diverges from the correct path. Coding constraints are frequently employed in order to limit burst error lengths, so that the trellis (path memory) can be made with a practical maximum number of states. However, in any sequence detector, such as a Viterbi detector, the trellis will have multiple states and must receive multiple samples before it can reach its decision as to each most likely path, and therefore each most likely binary data value (one or zero) to put out.
Examples of magnetic recording and playback channels employing PRML are found in commonly assigned U.S. Pat. No. 5,521,945 to Knudson, entitled: xe2x80x9cReduced Complexity EPR4 Post-Processor for Sampled Data Detectionxe2x80x9d; and U.S. Pat. No. 5,844,738 to Behrens et al., entitled: xe2x80x9cSynchronous Read Channel Employing a Sequence Detector with Programmable Detector Levelsxe2x80x9d. A paper by R. Behrens and A. Armstrong, entitled: xe2x80x9cAn Advanced Read/Write Channel for Magnetic Disk Storagexe2x80x9d, IEEE Twenty-Sixth Asilomar Conf. on Signals, Systems and Computers, Vol. 2, pp. 956-960, October 1992, also provides useful background information concerning a number of issues relating to PRML.
In magnetic data storage devices, such as hard disk drives for example, user data is typically stored in blocks or sectors defined within a data track. Tracks may be a single spiral track as in optical recording, or may be a multiplicity of discrete concentric tracks as is the practice in magnetic disk recording. Each data sector typically begins with certain overhead information which may include a synchronization field, an address mark pattern enabling data blocks to be properly framed, a data field of user data bytes and ECC syndrome bytes, and a pad field. Since the sequence detector uses path metrics and multiple states in arriving at each data decision, it has heretofore been necessary to include sufficient pad bits in the pad field at the end of each sector or data block in order to propagate the last user data or ECC sample through the detector trellis. The number of pad bits required to flush the path memory of the sequence detector depends primarily upon the Euclidean distance properties of the target model. If the detector pad could be reduced in size, each data sector could be made smaller, enabling a greater number of sectors to be recorded on the magnetic disk surface.
Therefore, a hitherto unsolved need has remained for a method for reducing the amount of sequence detector pad in a data block format without loss of user data and in a manner overcoming limitations and drawbacks of prior designs and methods.
A general object of the present invention is to reduce the length of a sequence detector pad pattern within a partial response, maximum-likelihood recording and playback channel in a manner overcoming limitations and drawbacks of the prior art.
Another object of the present invention is to provide a simple and elegant way to control a sequence detector to arrive at a most-likely decision as to a last binary value of a user data field without having to traverse an entire length of a path memory of the sequence detector.
A further object of the present invention is to provide a shortened detector pad for inclusion within a data pattern written to a signal-degrading storage medium, the pad length being chosen to be equal to, or less than, the number of bits in a state of a sequence detector. If the pad length equals the number of bits in a state, there will be only one state of the sequence detector corresponding to the pad pattern. If the pad length is less than the number of bits in a state there will be more than one sequence detector state corresponding to the pad pattern (typically a very small number of states), and the pad pattern is chosen so that paths that diverge from a common state of the sequence detector and reach the small number of states that correspond to the pad pattern will have a large separation from each other in Euclidean distance.
In accordance with one aspect of the present invention, a method is provided for reducing data format overhead in a storage device including a sequence detector having a series of states and a path memory of predetermined length. This method includes the steps of writing a predetermined shortened pad pattern at the end of a user data field pattern to a signal-degrading storage medium of the device; generating samples during read back of the user data field pattern and the shortened pattern; and controlling a sequence detector during receipt of samples of the shortened pad pattern to converge to one or a small number of predetermined states of the sequence detector during a convergence sequence having a length less than a sequence needed to traverse a length of a path memory of the sequence detector. The shortened detector pad pattern is selected so that paths that diverge from a common state of the sequence detector and reach the small number of predetermined states have a large separation from each other in Euclidean distance, thereby enabling accurate selection of a most likely path to the one state indicating a most likely estimate of a user data field last bit value. Most preferably, the storage device is a magnetic recording and playback device, and the signal-degrading storage medium comprises a magnetic storage medium such as a disk or tape, and the user data field pattern is scrambled to randomize a magnetic pattern recorded onto the magnetic storage mediuim, and the step of writing the predetermined shortened pad pattern is carried out without scrambling of the shortened pad pattern.
In accordance with another aspect of the present invention, a magnetic data storage device, such as a magnetic hard disk drive, or magnetic tape drive, implements partial response maximum-likelihood sampling data detection in a manner reducing data storage overhead. The device includes a write channel for writing a predetermined shortened pad pattern at the end of a user data field pattern to a magnetic storage medium of the device, and a read channel for generating synchronous samples during read back of the user data field pattern and the shortened pad pattern. The device further includes a sequence detector for determining sequences of most likely data symbols during receipt of samples and having a predetermined path memory length. The sequence detector includes control logic for controlling the sequence detector during receipt of samples of the shortened pad pattern so as to cause memory path convergence to only one or a small number of predetermined states of a series of possible detector states during a path convergence sequence having a duration less than duration of a path sequence needed to traverse a full length of the memory path of the sequence detector. The resultant state(s) can be used to obtain a most likely estimate of a user data field last bit value. Preferably, the control logic includes circuitry for eliminating any path along the detector memory path not leading to the resultant state(s). Also, the device""s write channel may include a precoder which puts out an output value Yk which is equal to Ykxe2x88x921, XOR Xk, where Xk is the input value. In this case the shortened pad pattern comprises a sequence of binary zeros selectively applied to the input of the precoder during a writing sequence for writing the shortened detector pad pattern.